Monolithic image chip for near-to-eye display

ABSTRACT

A set of light emitting devices can be formed on a substrate. A growth mask having a first aperture in a first area and a second aperture in a second area is formed on a substrate. A first nanowire and a second nanowire are formed in the first and second apertures, respectively. The first nanowire includes a first active region having a first band gap and a second active region having a second band gap. The first band gap is greater than the second band gap. The second nanowire includes an active region having the first band gap and does not include, or is adjoined to, any material having the second band gap.

FIELD

The present disclosure relates to arrays of light emitting diodes (LEDs)and in particular to arrays capable of emitting light of at least twodifferent wavelengths. It also relates to methods of fabricating sucharrays. Further, the disclosure provides a monolithic substrate havingan array of emissive subpixels of at least two types, with each typeemitting light of a different color. Such a substrate is termed an imagechip. With three subpixel types and corresponding colors that areapproximately red, green, and blue, the subpixel array on the image chipis capable of displaying full-color images.

BACKGROUND

For light emitting devices, such as light emitting diodes (LED), theemission wavelength is determined by the band gap of the active regionof the LED together with thickness determined confinement effects. Oftenthe active region includes one or more quantum wells (QW). ForIII-nitride based LED devices, such as GaN based devices, the activeregion (e.g., quantum well) material is preferably ternary, such asIn_(x)Ga_(1-x)N, where 0<x<1.

The band gap of such III-nitride is dependent on the amount of Inincorporated in the active region (e.g., in the QW(s)). Higher Inincorporation will yield a smaller band gap and thus longer wavelengthof the emitted light. As used herein, the term “wavelength” refers tothe peak emission wavelength of the LED. It should be understood that atypical emission spectra of a semiconductor LED is a narrow band ofwavelength centered around the peak wavelength.

Multi-color LED arrays in the prior art suffer from several drawbacks.Some multi-color LEDs form nanowire LEDs which emit different colorlight (i.e., different wavelength) from different portions of the samenanowire, which makes it difficult to control and selectively activatethe different emission wavelengths from the same nanowire LED.

SUMMARY

Embodiments include a semiconductor structure including light emittingdevices includes a first light emitting device containing a firstnanowire located on a substrate, where the first nanowire includes afirst active region, the first active region including a first loweractive region having a first band gap and a first upper active regionhaving a second band gap, where the first band gap is greater than thesecond band gap, and a second light emitting device containing a secondnanowire located on the substrate, where the second nanowire includes asecond active region having the first band gap and does not include, noris in physical contact with, any material having the second band gap.

Further embodiments include a method of making a semiconductor structureincluding light emitting devices that includes forming a growth maskhaving a first aperture in a first area and a second aperture in asecond area on a substrate, forming a first nanowire through the firstaperture, where the first nanowire includes a first active regionincluding a first lower active region and a first upper active region,the first lower active region having a first band gap and the firstupper active region having a second band gap, where the first band gapis greater than the second band gap, and forming a second nanowirethrough the second aperture, where the second nanowire includes a secondactive region having the first band gap and does not include, nor is inphysical contact with, any material having the second band gap.

Further embodiments include an image chip that includes a semiconductorstructure including light emitting devices such as described above,where the substrate of the semiconductor structure is a monolithicsubstrate and the the light emitting devices constitute an array ofemissive subpixels of at least two types located upon the monolithicsubstrate. Further embodiments relate to a method of making an imagechip including a semiconductor structure including light emittingdevices such as described above.

Further embodiments include a device that includes a support includingat least a first area and a second area, and a plurality of first lightemitting devices located over the first area of the support, each firstlight emitting device containing a first growth template including afirst nanostructure, and each first light emitting device has a firstpeak emission wavelength. The device also includes a plurality of secondlight emitting devices located over the second area of the support. Eachsecond light emitting device contains a second growth template includinga second nanostructure, and each second light emitting device has asecond peak emission wavelength different from the first peak emissionwavelength. Each first growth template differs from each second growthtemplate.

The support may be a growth substrate or a handle substrate which isattached to the device after the device is grown. Each firstnanostructure may comprise a first nanowire core which comprises aninner portion or an entirety of the first growth template, each firstnanowire core protrudes through a first aperture in a growth mask in thefirst area. Each second nanostructure may comprise a second nanowirecore which comprises an inner portion or an entirety of the secondgrowth template, each second nanowire core protrudes through a secondaperture in a growth mask in the second area. Preferably, each firstgrowth template comprises the first nanowire core and at least one firstgrowth template layer around the first nanowire core, such that thefirst growth template layer extends laterally beyond the first apertureover the growth mask, and each second growth template comprises thesecond nanowire core and at least one second growth template layeraround the second nanowire core, such that the second growth templatelayer extends laterally beyond the second aperture over the growth mask.

Each first and second nanowire core may comprise a first conductivitytype (e.g., n-type) semiconductor material. A first active region may belocated around each first nanowire core, the first active regioncomprising at least one first quantum well having a first band gap. Asecond active region may be located around each second nanowire core,the second active region comprising at least one second quantum wellhaving a second band gap different from the first band gap. Preferably,each first and second nanowire core comprises a III-nitridesemiconductor material (e.g., gallium nitride), and each first andsecond quantum well comprises an indium gallium nitride material.

A first junction forming element comprising a semiconductor material ofa second conductivity type (e.g., p-type) different from the firstconductivity type may be located around each first active region to forma p-n or p-i-n junction. A second junction forming element comprising asemiconductor material of the second conductivity type different fromthe first conductivity type may be located around each second activeregion to form a p-n or p-i-n junction.

Each first growth template may differ from each second growth templateby at least one of: (a) a growth area for a respective active region,(b) a ratio of exposed growth planes, or (c) a spacing from adjacentgrowth templates. Each first quantum well preferably contains adifferent amount of indium from each second quantum well due to thedifference between the first and the second growth templates. Forexample, each first growth template may have a nanopyramid shape andeach second growth template may have a nanopillar or nanowire shape.Thus, each first growth template may have a larger p-plane facet areacontacting the first active region than a p-plane facet area of thesecond growth template contacting the second active region. In thisexample, this may lead to each first quantum well containing a higheramount of indium and a lower peak emission wavelength than each secondquantum well due to the difference in the p-plane facet area between thefirst and the second growth templates.

In another embodiment, each first aperture has a substantially equalwidth or diameter to each second aperture, each first aperture is spacedfarther apart from adjacent first apertures than each second aperture isspaced apart from adjacent second apertures, and each first growthtemplate has a substantially equal or smaller growth area contacting thefirst active region than a growth area of each second growth templatecontacting the second active region.

In another embodiment, a plurality of third light emitting devices arealso located over a third area of the support, each third light emittingdevice has a third peak emission wavelength different from the first andthe second peak emission wavelengths. Each third light emitting devicemay contain a third growth template including a third nanostructure,which may have a nanopyramid shape. Each third nanostructure maycomprise a third nanowire core which protrudes through third aperture ina growth mask in the third area, each third aperture may have asubstantially equal width or diameter to each first and second aperture,and each third aperture may be spaced farther apart from adjacent thirdapertures than each first and second apertures are spaced apart fromadjacent respective first and second apertures. Each third growthtemplate may have a substantially equal or smaller growth areacontacting a third active region than a growth area of each first andsecond growth template contacting the respective first and second activeregions, and the third peak emission wavelength may be longer than thefirst and the second peak emission wavelengths.

In another embodiment, each first aperture may have substantially largerwidth or diameter than each second aperture, each first aperture may bespaced substantially equal or farther apart from adjacent firstapertures than each second aperture is spaced apart from adjacent secondapertures, and each first growth template may have substantially thesame or a smaller growth area contacting the first active region than agrowth area of each second growth template contacting the second activeregion.

The first and the second light emitting devices may comprise lightemitting diodes, and each first and second junction forming therespective light emitting devices may be selected from a semiconductorshell, a continuous semiconductor layer which contacts plural growthtemplates, or a continuous semiconductor layer with interstitial voidswhich contacts plural growth templates.

Another embodiment provides a method of making a light emitting devicethat includes providing a growth substrate including a growth maskhaving a plurality of first apertures in a first area and a plurality ofsecond apertures in a second area; and selectively growing a pluralityof first nanostructures through the first apertures and a plurality ofsecond nanostructures through the second apertures in a samenanostructure growth step, where the first and the second nanostructuresinclude an inner portion or an entirety of respective first and secondgrowth templates. The method also includes growing first and secondactive regions on respective first and second growth templates in thesame active region growth step, and growing first and second junctionforming elements on respective first and second active regions in a samejunction forming element growth step to form respective first and secondlight emitting devices. Each first growth template differs from eachsecond growth template such that each second light emitting device has asecond peak emission wavelength different from a first peak emissionwavelength of each first light emitting device.

Another embodiment provides an intermediate semiconductor structure,including a substrate, a plurality of first growth templates including afirst semiconductor nanostructure located over a first area of thesubstrate, and a plurality of second growth templates including a secondsemiconductor nanostructure located over a second area of the substrate.Each first growth template differs from each second growth template byat least one of: (a) a growth area for a respective active region, (b) aratio of exposed growth planes, or (c) a spacing from adjacent growthtemplates.

Another embodiment provides method of making a semiconductor device,including providing the above described intermediate semiconductorstructure, and growing first and second indium gallium nitridesemiconductor active regions on respective first and second growthtemplates in a same active region growth step. Each first active regioncontains a different amount of indium from each second active region dueto the difference between the first and the second growth templates.

Another embodiment provides a method of growing a III-V semiconductornanowire, including growing the III-V nanowire over a substrate by MOCVDin a group V limited growth regime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show the top view and a side cross sectional view alongline A-A, respectively, of a semiconductor device of an embodiment ofthe disclosure.

FIGS. 2A, 2C, 3A, 4A, 5A, 6A and 8A show top views of a semiconductordevice of alternative embodiments of the disclosure. FIGS. 2B, 2D, 3B,4B, 5B, 6B and 8B show side cross sectional views along line A-A ofrespective FIGS. 2A, 2C, 3A, 4A, 5A, 6A and 8A.

FIG. 7 shows aside cross sectional view of a semiconductor deviceaccording to various embodiments of the disclosure.

FIGS. 9A and 9B are theoretical plots of nanowire length versus maskaperture spacing and mask aperture diameter, respectively.

FIG. 10 illustrates an image chip architecture according to anembodiment. In this example, a 6-channel cathode yields interconnectpitch of about 10 microns.

FIG. 11 illustrates examples of a common-cathode pixel array and amultichannel-cathode pixel array.

FIG. 12 illustrates green and blue subpixels created from core-shellnanowires having green-emitting and blue-emitting portions of the activeregion.

FIG. 13 illustrates green and blue subpixels wherein subpixels ofdifferent colors have different geometries formed by the growth process.The structural difference between the subpixels gives rise to thedifference in emitted color. Contact layers are not shown.

FIG. 14 is a circuit diagram for a common-cathode image chip.

FIG. 15 is a circuit diagram for two-channel-cathode image chip.

FIG. 16 is a top-down schematic depiction of several subpixels within atwo-channel cathode image chip. The chip has vertical cathode lines andhorizontal anode interconnects.

FIG. 17 is a cross-section schematic depiction of several subpixels in atwo-channel cathode image chip. Light emitted by the subpixels may beextracted through the substrate. For simplicity, each subpixel isillustrated as including a single nanowire.

DETAILED DESCRIPTION

As discussed above, the present disclosure is directed to arrays oflight emitting diodes (LEDs), arrays capable of emitting light of atleast two different wavelengths, methods of fabricating such arrays, andan image chip. The drawings are not drawn to scale. Multiple instancesof an element may be duplicated where a single instance of the elementis illustrated, unless absence of duplication of elements is expresslydescribed or clearly indicated otherwise. Ordinals such as “first,”“second,” and “third” are employed merely to identify similar elements,and different ordinals may be employed across the specification and theclaims of the instant disclosure. As used herein, a first elementlocated “on” a second element can be located on the exterior side of asurface of the second element or on the interior side of the secondelement. As used herein, a first element is located “directly on” asecond element if there exist a physical contact between a surface ofthe first element and a surface of the second element.

As used herein, a “layer” refers to a material portion including aregion having a substantially uniform thickness. A layer may extend overthe entirety of an underlying or overlying structure, or may have anextent less than the extent of an underlying or overlying structure.Further, a layer may be a region of a homogeneous or inhomogeneouscontinuous structure that has a thickness less than the thickness of thecontinuous structure. For example, a layer may be located between anypair of horizontal planes between, or at, a top surface and a bottomsurface of the continuous structure. A layer may extend horizontally,vertically, and/or along a tapered surface. A substrate may be a layer,may include one or more layers therein, and/or may have one or morelayer thereupon, thereabove, and/or therebelow.

Herein “p-plane” means “pyramid plane” and represents a {1101} plane inthe III-nitride system, “c-plane” represents a {0001} plane, and“m-plane” represents a {1010} plane. Growth rate means layer growth ratewhen not otherwise specified. Usually, in selective growth, volumegrowth rate is constant irrespective of geometry. One aspect of thedisclosure described below provides the ability to locally set a grownvolume of semiconductor material.

Herein “kinetically limited growth regime” means a growth regime wheregrowth rate predominantly is hindered by an energetic barrier (lowtemperature, cracking of source material, release of surface bonds etc.)to reach thermodynamic equilibrium. “Mass flow limited growth regime”means a growth regime where precipitation at the growth area ispredominantly unhindered but growth rate is limited by concentration ofsource material at the growth area. “Group III limited growth regime”means a growth regime that is mass flow limited with regards to thecolumn III element or elements while “group V limited growth regime”means a growth regime that is mass flow limited with regards to thecolumn V element or elements.

A growth system is usually limited by a subset of these parameters incombination; i.e., a growth rate is often limited by sourceconcentration even if a severe kinetic reaction barrier exists in thesystem. The relative weight of V to III limited growth rate is mosteasily adjusted by altering the total V/III-ratio. The relative weightof kinetical to mass flow limited growth rate is more complex, as itdepends on the origin of the kinetical barrier. Usual parameters thatchange this are temperature, total pressure, total flow, carrier gas andV/III-ratio. It is also important to realize that in V/III growth where50% of each element group is needed to proceed with precipitation, onegroup element can be kinetically limited while the other group elementsare mass flow limited such that only one of the groups is causing thegrowth rate limitation. In traditional V/III growth, in MOVPE andsimilar growth methods, high quality material is often grown with a highoverflow of V material, for example, in GaN using V/III source gas massflow ratio of at least 1,000, such as 1,000 to 10,000. Under theseconditions, the growth rate is group III limited while the group Vconcentration is high to keep a saturated overflow of V material to thegrowth surface. In contrast, in the group V limited growth mode, theV/III ratio is preferably less than 1,000, such as 0.001 to 100, forexample less than 1, such as 0.001 to 0.1.

To determine the type of growth rate limitation at a certain growthcondition is relatively straightforward and is done by changing thecondition parameters and recording the change in growth rate (e.g.,thickness measurements). Energy heights of kinetic barriers can bededuced by temperature dependent growth rate measurements.

Note that the kinetic barrier of cracking efficiency of NH₃ due to GaNis well established but is an additional parameter, not discussed in thetheoretical part below. The theoretical part treats flow rate and growthareas as purely geometrical and volume impact factors on growth rate.

Simultaneous growth of several color LEDs in one step would be of highcommercial interest, not only for RGB (Red Green Blue), YB (Yellow Blue)or YGB (Yellow Green Blue) combinations for white rendition of light(i.e., white light emitting LED based on combination of RGB, YB or YGBpeak wavelength emission) but also high efficiency GB (Green Blue) sinceviable green phosphors and green LEDs based on other material systemshave been hard to realize. In view of the shortcomings of prior artdevices, the present inventors have devised a selectively grownstructure that can be used to form optoelectronic devices, e.g., LEDarrays. As used herein, the term simultaneous growth in one step meansthat the corresponding layers or structures of different color emittingLEDs are grown in one step. Thus, for example, the nanostructure coresof different color emitting LEDs may be grown in the same first step,the QW active regions of different color emitting LEDs may be grown inthe same second step and the junction forming elements or shells ofdifferent color emitting LEDs may be grown in the same third step.

Preferably, the arrays of semiconductor LED elements includenanostructured (e.g., nanowire or nanopyramid) cores, herein referred toas templates, surrounded by shells comprising active region and volumeelement shells. Such nanostructure LEDs may be considered as “pointsources” of light or UV radiation, which are different from the priorart LED structures comprising elongated stripes or planar bulksemiconductor layers. The templates may comprise a single grown layer,such as a nanowire core, but also be formed from multiple layers as willbe described below.

In some of the following embodiments, the group V limitedconditions/regime are exemplified in a nanowire core growth step throughan opening in a mask. However, any other suitable growth regime wherethe group V limitation is achieved with sufficiently good qualitymaterial as product, can be utilized, such as VLS growth or otherselective growth methods. Thus, the selective nanowire growth as thegroup V limited growth step is therefore used to merely exemplify ratherthan limit the disclosure.

In one embodiment, the arrays include at least two semiconductorstructures, such as nanostructures, having different properties in termsof geometry, e.g., length and/or width of individual elements, and/orthe spacing between different elements. Also, the area of the quantumwell and the relative facet distribution on the template which the QWare grown on may differ. Conveniently, in examples below, particular,elements with different geometrical properties are grouped togetheraccording to selected patterns in order to provide the desired effect ofmulti-color light (i.e., multiple peak wavelength) emission. Elements orgroups of elements, with different optical and geometrical propertiesmay also be fabricated in intermixed patterns, i.e., in order to blendcolors at chip level or to facilitate other optical interactions.

In another embodiment, a method of making the multi-color LED arrayincludes forming a growth mask containing a pattern of growth apertureshaving different size (area and/or diameter) and/or different spacingbetween apertures in different portions of the mask in order to providethe different geometries of the semiconductor structures in the array.The method utilizes in situ growth of multiple groups of semiconductorstructures (e.g., nanowire cores) without the need for other processsteps outside of the growth chamber, such as etching and ionimplantation, during the fabrication of the semiconductor structures. Insome embodiments, the method comprises a plural step growth mode,including at least one nanowire core growth step and one or more stepsof growing radial layers around the cores to form a template.

An advantage of a method of making a multi-color LED array according tovarious embodiments may include the opportunity to use small footprintaperture sizes to fabricate larger semiconductor structures, formingdevices distinguished by appreciably smaller cross sectional foot printarea compared to the semiconductor structure total base area, the smallfoot print giving low crystal defect or dislocation density as comparedto the substrate. Another advantage of a method of making a multi-colorLED array according to various embodiments may include that a variety ofstructure shapes and relative facet area ratios ma be fabricated at thesame time (i.e., in the same growth step). By introducing a growthregime where growth area does not change until the transition to radiallayer growth, height of the structures, and therefore resulting shape ofstructures may be made independent of one of the aperture size and/orspacing as will be described below.

A method according to various embodiments may also include utilizing ananowire growth regime where the resulting lengths of the nanowiresmaybe controlled to be independent of or dependent on mask geometry in acontinuous manner to optimize different wavelength emission by differentLEDs in the array.

Various embodiments may enable the fabrication of groups ofsemiconductor structures (e.g., nanostructure cores) with similar heightbut different shape (e.g., different thickness nanowires and/ornanowires and nanopyramids). Various embodiments may also enablefabrication of groups of semiconductor structures with different heightbut similar shape.

Various embodiments may provide the ability to fabricate groups ofsemiconductor structures comprising a layer having approximately thesame shape but different total area. Embodiments may further enablefabrication of groups of semiconductor structures comprising a layerwith approximately the same area but different shape.

Various embodiments may provide the ability to fabricate groups ofsemiconductor structures comprising a layer with approximately the samearea but different relative facet area ratio between groups. Embodimentsmay further enable fabrication of groups of semiconductor structureswith similar spacing, comprising a layer with different relative facetarea ratio between groups.

In a general aspect, various embodiments of the present disclosurerelate to selectively grown optoelectronic structures or elements,provided on a substrate, where groups of a plurality of individualselectively grown structures may have different properties in terms ofsize, geometry, spacing and distribution over the substrate to providefor different color light emission from the different groups. In termsof geometry, the individual elements may have the same lengths and/orheights or have different lengths and/or heights, they may havedifferent or the same effective diameter (or width), they may exhibitdifferent facets, and the overall geometry may be varied based onspacing between or size of the apertures of the mask used for the growthof individual elements and between groups of elements.

Thus, embodiments of the disclosure may include a substrate and at leasttwo growth elements (e.g., nanostructure cores) protruding from thesubstrate. The cores may protrude from the substrate through aperturesin a growth mask over the substrate. The nanostructure cores may have alateral extension (i.e., diameter or width) of 3 μm or less in anylateral direction, for example 100 nm to 1 micron, and a height between50 nm and 10 μm, such as 0.1 to 5 microns. Examples of nanostructurecores include nanowire (i.e., nanopillar) and nanopyramid cores that aredescribed below. As noted above, the nanostructure core itself or ananostructure core with one or more shell layers forms the growthtemplate. Furthermore, if desired, the growth substrate may be removedafter the growth of the cores over the substrate, such that the finisheddevice is a freestanding device or is mounted to a different handlesubstrate.

For the InGaN quantum well (“QW”) active region, the higher growth rateof the InGaN semiconductor layer (e.g., QW shell on a semiconductornanostructure core) results in higher indium incorporation into thequantum well. A higher indium incorporation results in a narrower bandgap of the QW semiconductor material and thus in a longer emitted peakwavelength by the LED (i.e., a red shift in the emitted color).

In a first embodiment, the higher QW growth rate occurs on first growthareas (e.g., on nanowire or nanopyramid templates) that are spacedfarther apart from each other by non-growth areas (e.g., maskedsubstrate areas) than on second growth areas (e.g., templates) which arespaced closer to each other. Thus, in the first embodiment, a firstgrowth area contains semiconductor templates which are spaced apart fromadjacent templates by a larger distance than the adjacent templates in asecond growth area. This difference in spacing results in higher indiumincorporation in the active regions (e.g., QW(s)) formed on thetemplates in the first growth area than on the templates in the secondgrowth area, even when the active regions are formed in the same growthstep at the same time in both first and second groups of growth areas.The LEDs formed in the first growth area have a longer peak emissionwavelength than LEDs formed in the second growth area.

The wider spaced templates in the first growth area may have a largerheight and thus a larger exposed surface area than the narrower spacedcores in the second area in a typical Group III limited (e.g., mass flowlimited) MOCVD growth regime. The larger height and surface area of thecores in the first area means that less indium may be incorporated intothe active region formed on the templates in the first area than oncores in the second area. Thus, the difference in height and area of thecores works against the difference in spacing between the templates indifferentiating the emitted wavelength from the active region.

The present inventors have discovered that introducing a Group Vmaterial limited growth regime, in contrast to the traditional group IIIlimited regime used in III-V growth, may enable the difference in heightof the templates in the first and the second growth areas to be madesmaller or even eliminated (i.e., such that the height of the templatesin the first and second growth areas may be substantially equal). Thisalso means that the total growth area for subsequent layers will belarger in the second growth areas. Therefore, in embodiments, thetemplates of the first embodiment are formed through apertures in thegrowth mask by at least one growth step, such as a at least one metalorganic chemical vapor deposition (MOCVD) growth step operating in theGroup V material limited growth regime. In this regime, the growth isGroup V element (e.g., nitrogen) limited because a relatively low GroupV to Group III source gas ratio (e.g., ammonia:TEG or TMG ratio below1,000, such as 0.001 to 100, for GaN cores) is used. In should be notedthat for templates that include both a nanostructure (e.g., nanowire)core and one or more shells, the nanostructure core may be grown in thegroup V limited mode, while the shell(s) may be grown in either thegroup III or group V limited mode, as will be illustrated below. Thus,the template (e.g., nanowire core or nanowire core plus at least oneshell) height and area does not work against the template spacingdifference in ensuring a different emission wavelength for LEDs in thefirst and the second areas. The more or less instantaneous gas phasediffusion of ammonia (as compared to TEG or TMG) makes it a source thatdoes not decrease in gas phase concentration locally when the growth isgroup V (e.g., ammonia) limited, as will be discussed in more detailbelow.

In a second embodiment, more indium is incorporated into the activeregions (e.g., QWs) grown on p-planes of the templates than the activeregions grown on m-planes of the templates. Thus, in the secondembodiment, the templates in a first growth area contains more exposedp-plane facet area (and less m-plane facet area) than the templates in asecond area. In other words, at least two of the templates havedifferent areal facet ratio or shape. The areal facet ratio is therelative area of at least two such facets, such as a p-plane facet andan m-plane facet.

For templates that are formed by growth through apertures in the mask,the mask in the first growth area may contain larger apertures than themask in the second growth area. This may result in nanopyramid templateswith larger area of exposed p-planes to be formed in the first growtharea and nanowire templates with smaller area of exposed p-planes (e.g.,p-planes exposed only at the tip of the nanowire) and larger exposedm-planes (e.g., exposed on the sidewalls of the hexagonal cross-sectionnanowire templates) to be formed in the second growth area. Thisdifference in amount of exposed p-plane area on the templates results inhigher indium incorporation in the active regions (e.g., QW(s)) formedon the templates in the first area than on the growth areas in thesecond area, even when the active regions are formed in the same growthstep at the same time in both first and second growth areas. Therefore,the LEDs formed in the first growth area will have a longer peakemission wavelength than LEDs formed in the second growth area.

The active region growth rate on larger growth areas is slower thangrowth rate on smaller growth areas. Thus, less indium is incorporatedinto the active regions formed on larger area templates than on smallerarea templates (unless spacing is changed to the extent to give a higherimpact on the total local growth area than the relative difference intemplate size). Thus, if the p-plane facet dominant templates (e.g.,nanopyramid templates) have a smaller area or height than the m-planefacet dominant templates (e.g., nanowire templates), then less indiumwould be incorporated into the m-plane facet dominant templates, whichenhances the positive effect of the difference in the exposed p-planearea between the nanopyramid and nanowire templates (i.e., total areadifference works synergistically with the difference in exposed p-planearea).

The present inventors have also discovered that increasing local growtharea, such as by increasing the density of apertures, may increase theheight of the densely grown cores further as compared to cores furtherspaced apart when growing in a group V limited regime. Without wishingto be bound by theory, this effect may be explained by the groupV-source gas, especially and here exemplified by NH₃, having a severekinetic bottleneck in cracking to elemental nitrogen. Approximately 15%NH₃ is cracked at 950° C. catalyzed by a GaN surface. A rough estimationof gas phase cracking in the absence of GaN or exposed to a dielectricsurface, such as a silicon nitride mask, is 1% to 3% at typical GaNgrowth temperatures, such as between 800 and 1000° C. The enlargedsurface area of GaN templates formed by a denser spacing of cores can inthis way enhance V-limited growth rate and the height of the closerspaced templates well above the height of the templates spaced furtherapart from each other. This ensures that the total area of the closerspaced templates in the second growth area are larger than those of thefarther spaced templates in the first growth area. Thus, in embodimentsit may be desirable to use a V-group limited growth regime to ensurethat the templates in the second growth area have a larger area orheight than the templates in the first growth area, such that moreindium may be incorporated into the active regions grown in the secondfirst growth area. Therefore, in embodiments, the III-nitride nanowireor nanopyramid templates of the second embodiment may be formed throughapertures in the growth mask by a metal organic chemical vapordeposition (MOCVD) method operating in the group V limited regime incombination with sufficient density of apertures to produce an increasein GaN caused catalytic cracking of the NH₃ group V source gas. This mayresult in taller and larger area templates in the second growth areathan in the first growth area.

A third embodiment may be a combination of the first and the secondembodiments, such that the apertures in the growth mask may be larger inthe first growth area than in the second growth area, and the aperturesmay be farther spaced apart in the first growth area than in the secondgrowth area. Thus, the aperture spacing and size may synergisticallyincrease the difference in peak emitted wavelength between LEDs formedin the first and the second growth areas.

In this embodiment, the heights of the templates in first growth areamay be larger than the height of the templates in the second growtharea. This effect may be enhanced by the increase of the GaN surfacearea in the first growth area, utilizing the kinetic barrier of NH₃cracking as described above. The nanowire core growth step comprised inthe template growth in the third embodiment may then preferably beconducted in the group V material limited growth regime, unless thedifference in aperture spacing is much greater than the difference inaperture size between the first and the second areas. In this case, agroup III limited growth regime may preferably be used. It should benoted that the formation of the active region (e.g., QW) and shellgrowth may be conducted in the Group III limited growth regime in allthree embodiments irrespective of the regime used to grow the templates.

Thus, in various embodiments, the mask used for growing the templatesfor the LEDs or other optoelectronic devices may contain apertures ofdifferent spacing and/or size in different growth areas to promotegrowth of different shape templates in each growth area, andsubsequently the growth of active regions with different composition(e.g., different indium content) on said templates.

The term selective growth refers to epitaxial growth on at least onesurface where growth on other surfaces is low or even negligible.Selective nanostructure template growth by MOCVD preferably uses a maskwith openings over an epitaxial substrate where growth is confined tothe apertures with no growth on the mask. However, other forms ofnanostructure (e.g., nanowire) growth, such as VLS, or particle enhancedgrowth, are also selective in the way that one direction or facet of thecrystal is growing appreciably faster than other facets. Nanowire growthcould be called surface selective growth, but the term surface selectivegrowth has also been used for any kind of structure where one surface orfacet is growing faster than other surfaces. Thus, the presentdisclosure is not limited to templates growth through mask apertures andincludes various other types of nanostructure templates. Furthermore,the growth method is not limited to MOCVD and includes other methods,such as CBE, MBE, LP-MOVPE, etc. Any method where effective anddistinctive selective growth regimes can be realized is viable.

Preferably, the optoelectronic device is an LED array which hastemplates of different shape, size and/or pitch. Preferably eachtemplate is a semiconductor template of a first conductivity type (e.g.,an n-type GaN nanowire or nanopyramid template). The template, theactive region (e.g., InGaN quantum well(s)) and the shell of a secondconductivity type (e.g., a p-type GaN shell) form a p-i-n junction. Theactive region emits light (e.g., visible light or UV radiation) when avoltage is applied over the junction. The light emitted from at leasttwo LEDs in the array has different wavelengths. The differentwavelengths of the emitted light are achieved by at least two templatesexhibiting different area.

Various non-limiting embodiments of the above described mask apertureand nanostructure template arrangements will be described with referenceto FIGS. 1 to 6. FIGS. 1A, 2A, 2C, 3A, 4A, 5A and 6A show the top viewof the nanostructure templates in the mask apertures, while FIGS. 1B,2B, 2D, 3B, 4B, 5B and 6B show the respective side cross sectional viewsof the devices in respective FIGS. 1A, 2A, 2C, 3A, 4A, 5A and 6A alongline A-A in each figure. Three different variables are varied in anumber of combinations shown in these figures, namely 1) the spacingbetween apertures and groups of apertures; 2) the mask aperture size;and 3) whether one of the growth steps comprising the template growth(e.g., template nanostructure core growth) is group V or group IIIlimited.

FIGS. 1A and 1B illustrate the first embodiment where differently spacedtemplates are providing for LEDs emitting two different wavelengths. Inthese figures, a growth mask 6 (e.g., a silicon nitride layer or anotherinsulating material layer) is grown over a substrate 5 (e.g., silicon,GaN, sapphire, etc.). The substrate 5 may contain a semiconductor bufferlayer (e.g., GaN, AlGaN, etc. not shown for clarity) under the mask 6.FIG. 1A is a schematic top view of an LED chip with two groups ofnanostructure templates 2A, 2B in respective first 8A and second 8Bgrowth areas, and FIG. 1B is a side cross section view with across-section through the elements along the line A-A in FIG. 1A. Asshown in the figures, there may be plural growth areas 8A, 8B on thesubstrate 5 (e.g., alternating first and second growth areas).

In FIGS. 1A and 1B, the geometry of the templates 2A, 2B is controlledby mask 6 aperture 10A and 10B spacing. In this embodiment, theapertures 10A, 10B have the same size (e.g., same width, length and/ordiameter), while spacing between apertures are different in growth areas8A and 8B. By setting nanowire core growth to be group III-limited by arelatively large group V:group III flow ratio, the height of thetemplates in growth areas 8A and can be made larger than the templateheights in growth areas 8B. As shown in these figures, nanopyramid typetemplates 2A, 2B are selectively grown in the respective apertures 10A,10B in respective growth areas 8A, 8B, due to group III limitedconditions of radial (pyramidal) shell growth steps after the nanowirecore growth. Other template types (e.g., nanowire templates) may begrown if the size of the apertures 10A and 10B is reduced. No group Vlimited growth step is warranted in this embodiment.

The first apertures 10A in the first growth area 8A are more widelyspaced apart from adjacent apertures than the second apertures 10B inthe second growth area 8B. Therefore, the first nanopyramid templates 2Aprotruding from the first apertures 10A in the first growth area 8A aremore widely spaced apart from adjacent templates than the nanopyramidtemplates 2B protruding from the second apertures 10B in the secondgrowth area 8B.

Due to the different spacing between the templates, the incorporation ofIn into the active region (e.g., QW) that may subsequently be formed onthe templates 2A, 2B in the same active region growth step will bedifferent. Specifically, there will be more In incorporation into theactive regions on the first growth templates 2A than In incorporationinto the active regions on the second growth templates 2B during theactive region growth step. Thus, the band gap will be wider in theactive regions grown on the first growth templates 2A than on the secondgrowth templates 2B. In particular, the less densely arranged LEDsformed on the first growth templates 2A will emit longer wavelengthlight (e.g., red, yellow or green light) while the more densely arrangedLEDs on the second growth templates 2B will emit shorter wavelengthlight (e.g. blue light).

It should be noted that this is conditioned by that the surface area ofthe templates 2A, being higher than the 2B templates has not increasedas much as the interstitial mask area has increased with spacingdifference of 8A and 8B. The first nanopyramid templates 2A have alarger height and thus a larger exposed surface area than the secondnanopyramid templates 2B due to the column III limited growth regime.The larger height and surface area of templates 2A means that lessindium will be incorporated into the active region formed on templates2A than on templates 2B. Thus, the difference in height and area of thetemplates works against the difference in spacing between the templatesin differentiating the emitted wavelength from the active region.

Therefore, the inventors have discovered that it may be preferable togrow the templates in the Group V limited growth regime when thetemplates have a different spacing in different growth areas (but wherethe aperture size in the same in both growth areas). As shown in FIGS.2A and 2B, when the group V limited growth regime is used with the samesize apertures 10A, 10B, the templates 2A protruding through the widerspaced apart openings 10A in the growth mask 6 in area 8A may have ananopyramid shape, while the templates 102B protruding through thenarrower spaced apart openings 10B in the growth mask 6 in area 8B mayhave a nanowire (e.g., nanopillar) shape.

The nanopyramid templates 2A and the nanowire templates 102B have aboutthe same height and a similar exposed surface area when they are grownin the group V limited growth mode. This means that a small difference(if any) in exposed surface area of the templates 2A and 102B will nothave a large negative impact on the difference in amount of indiumincorporation into the active regions on the templates. Furthermore, thenanowire templates 102B have m-plane “m” facets exposed along themajority of their sidewalls and smaller p-plane “p” facets exposed neartheir tips. This means that the active regions formed on the nanopyramidtemplates 2A with larger area of exposed p-plane facets than thenanowire templates 102B will synergistically have more indiumincorporation than the active regions formed on the m-plane facets ofthe nanowire templates 102B. Therefore, the active regions formed onnanopyramid templates in areas 8A will emit an even longer wavelengththan the active regions formed in nanowire templates 102B in areas 8B.Thus, the Group V limited growth regime enhances the difference inemitted wavelength between the LEDs in areas 8A and 8B.

FIGS. 2C and 2D illustrate an alternative embodiment where the nanowiretemplates 102B have a greater height and greater exposed surface areathan the nanopyramid templates 2A when these templates are grown in thegroup V limited growth mode in combination with a sufficiently highdensity of apertures in area 8B and a sufficiently high growthtemperature to warrant an increase in the GaN mediated catalyticcracking of the group V source gas (e.g., NH₃) in the second growth area8B.

As discussed above, approximately 15% of the group V source gas, NH₃, iscracked at 950° C. due to catalization of the cracking by the GaNtemplate surface. A rough estimation of NH₃ gas phase cracking in theabsence of the GaN templates or when the gas is exposed to a dielectricsurface, such as a silicon nitride mask 6 surface, is 1% to 3% attypical GaN growth temperatures between 800 and 1000° C. The enlargedsurface area of GaN exposed to the ammonia gas in areas 8B resultingfrom a denser spacing of the GaN cores in areas 8B than in areas 8A canenhance the group V limited growth rate and the height of the closerspaced nanowire shaped templates 102B in areas 9B well above the heightof the pyramid shaped templates 2A in areas 8A which are spaced furtherapart from each other. This ensures that the total area of the templatesof the closer spaced templates 102B in the growth areas 8B are largerthan those of the farther spaced templates 2A in the growth areas 8A.Thus, it may be desirable to use V-group limited growth regime at asufficiently high temperature (e.g., at least 800° C., such as 800 to1000° C., for example 950 to 1000° C.) to ensure that the templates 102Bin the growth areas 8B have a larger area or height than the templates2A in the growth areas 8A because even more indium may be incorporatedinto the active regions grown in areas 8A relative to the indiumincorporated into the active regions grown in areas 8B. In other words,since the templates 2A have a smaller exposed area, are spaced fartherapart and have more exposed p-plane area, more indium is incorporatedinto the active regions grown on templates 2A in areas 8A than intoactive regions grown on templates 102B in areas 102B.

While only two repeating growth areas 8A, 8B are shown in FIGS. 1 and 2,more than two growth areas may be formed on the same substrate 5. Forexample, as shown in FIGS. 3A and 3B, three different growth areas 8A,8B and 8C may be formed on the substrate 5. In this example, the size ofthe apertures 10A, 10B and 10C in the mask 6 is the same in all threeareas 8A, 8B and 8C. However, the apertures 10C in area 8C are spacedapart wider than apertures 10A in area 8A, and the apertures 10A in area8A are spaced apart wider (larger pitch) than apertures 10B in area 8B.In the non-limiting embodiment, nanowire templates 102B are formed inarea 8B and nanopyramid templates 2A and 2C are formed in areas 8A and8C, respectively. Thus, the active regions of LEDs formed on templates102B in area 8B will have the shortest emitted wavelength (e.g., blue),the active regions of LEDs formed on templates 2A in area 8A will havethe intermediate emitted wavelength (e.g., green) and the active regionsof LEDs formed on templates 2C in area 8C will have the longest emittedwavelength (e.g., red). Of course more than three growth areas may beincluded (e.g., five to seven areas) to form active regions of LEDswhich will emit red, orange, yellow, green and blue wavelength light.

In an alternative embodiment shown FIGS. 4A and 4B, instead of varyingthe spacing between adjacent apertures in growth areas 8A and 8B, themask aperture 10A, 10B area or size may be varied between growth areas8A and 8B. The spacing between adjacent apertures is the same in bothgrowth areas 8A and 8B. Apertures 10A in area 8A are larger thanapertures 10B in area 8B.

In this embodiment, the growth regime is preferably set to be group IIIlimited by using a relatively small group III:V flow ratio. Thereby,since the growth initially is set for growth in the vertical direction,due to the growth area being larger for templates protruding from largerapertures 10A in area 8A than for templates protruding from smallerapertures 10B in area 8B, the templates in area 8A may be wider andshorter than templates in area 8B because for a given time equal volumesof material will deposit in the mask apertures 10A, 10B. Continuing orswitching to template growth in radial direction may yield the twodifferent template types. Nanowire or nanopillar templates 102B may beformed in smaller apertures 10B in area 8B, while nanopyramid templates2A may be formed in larger apertures 10A in area 8A. The nanopyramidtemplates 2A have larger amount of p-plane side facets “p” exposed toactive region growth than the nanowire templates 102B. The nanowiretemplates 102B have more m-plane side faces “m” exposed than thenanopyramid templates 2A. Since the p-plane side facets result in moreindium incorporation into the active regions grown on these facets thanm-plane side facets, the active regions grown on nanopyramid templates2A in area 8A will emit a longer wavelength light than the activeregions grown on the nanowire templates 102B in area 8B during the samegrowth step.

Furthermore, in the group III limited growth regime, the nanowiretemplates 102B may be taller and have a larger exposed surface area thanthe nanopyramid templates 2A. This means that more indium will beincorporated into the active regions formed on nanopyramid templates 2Athan on nanowire templates 102B. This synergistic effect furtherincreases the difference in emitted wavelength between the activeregions formed in area 8A on templates 2A (e.g., even shorterwavelength) and the active regions formed in area 8B on templates 102B(e.g., even longer wavelength). As discussed above, more than two growthareas with different aperture size (e.g., 3 to 7 areas) may be used.

In the third embodiment illustrated in FIGS. 5A and 5B, both theaperture spacing (according to the first embodiment) and the aperturesize (according to the second embodiment) are varied between the growthareas. Thus, the apertures 10A in growth area 8A are both spaced apartwider from adjacent apertures and have a larger size than the apertures10B in growth area 8B.

In FIGS. 5A and 5B, the templates may be grown in either the group IIIlimited growth regime or in the group V limited growth regime if spacingof apertures in area 8B is close enough to increase GaN surface areaduring the nanowire growth step (in order to improve kineticalefficiency of NH₃ cracking) Both these regimes result in shorter,farther spaced apart nanopyramid templates 2A protruding through theapertures 10A in area 8A and taller, closer spaced apart nanowiretemplates 102B protruding through the apertures 10B in area 8B. Asprovided in the prior embodiments, the active regions of the LEDs grownon templates 2A in area 8A will contain more indium and will emit lightof a longer wavelength than the active regions of the LEDs grown ontemplates 102B in area 8B.

In FIGS. 6A and 6B, the apertures 10A, 10B have the same size andspacing as in FIGS. 5A and 5B. However, the templates are grown in thegroup V limited growth regime but where NH₃ cracking is not appreciablycatalyzed by the GaN surface. This also results in farther spaced apartnanopyramid templates 2A protruding through the apertures 10A in area 8Aand closer spaced apart nanowire templates 102B protruding through theapertures 10B in area 8B. However, the templates 2A and 102B have aboutthe same height. As provided in the prior embodiments, the activeregions of the LEDs grown on templates 2A in area 8A will contain moreindium and will emit light of a longer wavelength than the activeregions of the LEDs grown on templates 102B in area 8B. However, thedifference in indium incorporation and emitted wavelength of the activeregions will generally be smaller than that for devices containing thedifferent height templates in FIGS. 5A and 5B. Therefore, the IIIlimited regime may be preferred for the third embodiment unless thedifference in aperture spacing is much greater than the difference inaperture size. The GaN surface is a changing variable (growth timedependent), both for setting the template to mask ratio in radial growthand as a catalytic surface. The longer the growth is continued in thenanowire growth mode, the larger the difference becomes for catalyticeffect (i.e., the longer the nanowire, the greater the available ammoniacracking area because both the top and sidewalls of the nanowire helpcrack the ammonia).

Any suitable MOCVD process conditions may be used to form the templates2, 102 shown in FIGS. 1A-6B. In one example, the MOCVD reactor pressureis 5-100 kPa (50-1000 mbar) and the reactor temperature is 500-1200° C.,preferably 900-1200° C. The Group III source gas may be TMG(trimethylgallium) or TEG (triethylgallium) having a flow rate of 0.12and 1.2 μmol/min or 0.5-10 sccm/min. The Group V source gas may beammonia having a flow rate of 0.2 to 10 sscm/min, such as 0.2 to 3sccm/min.

The growth process may be made III group limited or group V limited bychanging the Group III to Group V source gas flow ratio. Furthermore thegroup V limited regime may be manipulated to be more or less kineticallylimited locally by the total local GaN surface area. Providing anincreasing flow of Group III source gas where growth rate is notincreasing with increasing Group III source gas flow will result in agroup V limited growth (i.e., a high III:V ratio). Providing a lowerflow of Group III source gas where growth rate is increasing withincreasing Group III source gas flow will result in group V limitedgrowth (i.e., a low III:V ratio).

As shown in FIG. 7, each completed LED 1 may is formed on a substrate 5,which may comprise a sapphire, SiC, Si or GaN substrate containing anoptional GaN or AlGaN expitaxial buffer layer 7. The mask 6 may comprisea layer of silicon nitride or silicon oxide (20-50 nm in thickness)deposited by PECVD. Several techniques known in the art can be used toform the apertures 10A, 10B, such as electron beam lithography (EBL),nanoimprint lithography, optical lithography and reactive ion etching(RIE) or wet chemical etching methods. Preferably each of the aperturesare 10 to 500 nm, such as 50 to 200 nm in size (i.e., diameter), have anarea of 150 nm² to 0.5 microns², and are pitched (i.e., spaced) 0.5-5 μmapart. The templates 2 are selectively grown in the apertures 10 byMOCVD or another suitable method, comprising of group III or group Vlimited step that further may be kinetically or mass flow limited, asdescribed above. The templates 2 may comprise GaN or another III-nitridesemiconductor material doped with a first (e.g., n-type) conductivitytype dopant.

The active regions 4 (e.g., quantum well(s)) are grown on the templates2 by MOCVD or another suitable method, and the junction formingelement(s) 3, such as shells of the second (e.g., p-type) conductivitytype are grown on the active regions. Preferably, the active regions 4and the shells 3 are grown in the group III limited regime. The shells 3may comprise p-type GaN and the active regions may compriseIn_(x)Ga_(1-x)N quantum wells with a preselected ratio of In to Ga(i.e., where x varies between greater than zero and less than 1). InGaNhas flexibility of peak emission wavelength between approximately 375and 1100 nm based on the indium content. If desired, the quantum wellsmay comprise a quaternary material, such as InAlGaN or another suitablesemiconductor material. InAlGaN extends the material flexibility ofwavelength between approximately 200 and 1100 nm but introduces furtherchallenges for high quality growth. The electrodes are then formed inelectrical contact with the templates 2 and the shells 3 to complete theLED.

The arrays produced according to one aspect of the disclosure may belaid out such that groups (i.e., growth areas 8A, 8B, etc.) of LEDs 1with different active region 4 band gaps in each area due to differentIn content of the active region are electrically addressable separately.In other words, different drive voltages or current can be applied tothe different groups of LEDs in different growth areas. This isdesirable since if the same drive voltage would be applied to allelements either the efficiency of some devices would be too low or anexcess voltage would be applied to others, which would be inefficientfrom an energy usage point of view. Furthermore, some (e.g., one ormore) of the groups of LEDs may be turned on while some other (e.g., oneor more) groups of LEDs may be turned off to change the color of lightemitted by the LED array. This may be accomplished by forming separateelectrodes in contact with each LED group (e.g., with each growth area)and separately connecting the electrode pairs from each group to one ormore voltage or current sources.

In an alternative embodiment, the different color light emitting LEDs indifferent growth areas may be electrically connected in series. Forexample, one or more areas 8A may be connected in series with one ormore areas 8B.

In the fourth embodiment, each active region on a single template can bemade to emit a plurality of wavelengths. A non-limiting example is shownin FIGS. 8A-8B where each active region is capable of emitting threewavelengths. This is achieved by a layout of the mask 6 aperture 10pattern comprising rows of apertures 10. The apertures 10 in each row“r” are closely spaced to adjacent apertures, while there is arelatively large distance between rows “r”. For example, the distance“y” between adjacent apertures in the same row “r” is at least two timeslarger, such as 2-10 times larger than the distance “x” between adjacentapertures in adjacent rows. Preferably the distance x between rows isleast 2-3 times the width of the individual LEDs 1 shown in FIG. 7.

Thus, the LEDs 1 are provided in rows, where facets of individualtemplates are spaced from an adjacent template in the same row by anarrow distance “y” which retards/hampers In incorporation into theactive regions 4. Therefore, the portion 4B of the active region 4 onthe m-plane side facet of the template 2 that faces and adjacent LED 1in the same row will have the least amount of indium and therefore willhave the shortest emission wavelength (e.g., in the blue region). On theother hand, portions 4A of the active region 4 formed on the template 2m-plane side facets facing to the side of the row (i.e., facing anadjacent row) where there are no close neighboring LEDs will be prone toincorporate more In. Thus, portions 4A of the active regions will haveemission with a longer wavelength (e.g., in the green region) thanportions 4B. Finally, the portion 4C of the active region 4 formed onthe top p-plane facets of the templates 2 may emit the longestwavelength light (e.g., in the red region) due to the highest Inincorporation in the active region on p-plane facets. If this highindium incorporation on the top p-plane facets of the nanowire templates102 is not desired in the first through the third embodiments, then thetemplates 102 may be etched back to remove the sharp p-plane facet tipand to leave a planar c-plane top surface on the nanowire templates 102.The active region portions formed on the c-plane facets of the templates102 do not preferentially incorporate indium during deposition.

As described above, LED emission color differences are controlled bylocal geometry of the substrate and local shapes and dimensions oftemplates on the substrate. To achieve a high quality rendition of whitelight, a difference in wavelength of up to 200 nm between the shortestand longest wavelength may be needed. To give a good rendition ofblue/green, a difference in wavelength of up to 100 nm between theshortest and longest wavelength may be needed though lower differencesmay be advantageous.

While the devices are discussed in the context of multicolored LEDs, theLED array may comprise single color LEDs over a range of wavelengths.Furthermore, even though many examples are given with the traditionaldivision between blue, green and red, any multiple set of colors can beenvisioned. For example, close laying colors may be generated to providesubstantially continuous spectrum mimicking black body emission types oflight sources. Furthermore, the templates described above can be usedfor other devices, such as lasers, photodetectors, transistors, diodes,etc.

Multi-Step Selective Growth for Mask Independent Structure Design

As described above, when utilizing nanowire growth as first step fortemplates for 3D structures, the structure size and shape is stillcoupled to spacing and aperture size (local A_(g)/A_(m)) under Group IIIlimited conditions. However, in embodiments of the disclosure, utilizinggroup V limited conditions, when growth is not rate limited by the GroupIII source material delivery, constant growth rate independent ofspacing can also be achieved.

The same mechanisms as described above are true for mask aperture size.Aperture size will mainly affect template length and width, andespecially in Group III-source material limited growth regimes, thetemplate can be grown at a length independent of aperture size

Thus, two-step growth have additional advantages as it gives theopportunity to, decouple global monotonous facet development andfabricate differently shaped semiconductor structures at differentpositions by varying aperture spacing and aperture size. It should benoted that the transition to radial growth on the side facets of thetemplates means a redefinition of the growth area in the sense that thenanowire side facets together with the top area of the nanowire are thenused as growth area while the top area was the growth area duringnanowire conditions.

The function of the nanowire growth can then be understood to constitutea preparation of a template for defining growth area for the subsequentradial growth independently of spacing and aperture size, with muchhigher freedom than ordinary one-step growth where growth rate isdirectly dependent of spacing and aperture size and growth volume isconstant. The layers provided as part of or on the nanowire core form atemplate for the provision of an active layer, and depending on therelative height of the nanowire the ratio between pyramid facets andside facets of the layers can be varied. Spacing will then increase theratio of p/m plane at same template length, since a larger ratio ofcollection volume/mask area for each nucleation makes more materialavailable during radial layer growth to the nanowire. By grouping thetemplates in different ways, such as by spacing and aperture size,different properties (height, width, facet area etc.) are achievable,giving primarily different emission wavelengths when ternary QWs aregrown on these templates.

As described above, a method of making multi-color emitting LEDsincludes defining a growth mask with a desired aperture pattern on asubstrate. The growth mask can be a layer covering the substrate havingapertures of different size and/or different spacing, the openings beinggrouped according to certain rules for obtaining the desired propertiesdescribed above. The templates are selectively grown in growth regimessuch that the lengths of the templates can be essentially independent ofaperture spacing.

When templates have been grown as desired, i.e. a plurality nanowiresand/or nanopyramids exhibiting various combinations of lengths, widthsetc. at least one radial layer (e.g., active region and shell) are grownon the templates. As shown previously, the templates can also befabricated where height can be made dependent or independent on aperturepitch depending on growth regime. This is also true for aperture size.Aperture size will then mainly affect template length and width, butlength can be made independent of aperture size.

At extreme conditions, where group III flow is high, as in many nanowireconditions, growth with small A_(m), as with extremely large apertures,is however not feasible, and will result in low quality growth and oftenliquid III element droplets form on the surface.

As shown, in nanowire growth, when growth is not rate limited by theGroup III source material delivery, constant growth rate independent ofspacing can also be achieved.

The same mechanisms as described above are true for mask aperture size.Aperture size will mainly affect template length and width, andespecially in Group III-source material limited growth regimes, thetemplate can be grown at a length independent of aperture size.

The apertures in the mask will normally have circular shape althoughother shapes are possible, such as hexagonal, or rectangular. Maskaperture size can be in the range of 10 nm-500 nm (“effectivediameter”), or area 150 nm²-0.5 μm².

The length/width ratio of a template has a direct effect on thec-plane/p-plane/m-plane area ratio of the radial layer, such as theactive region QWs. Shorter templates give a larger pyramid part at thesame volume of the underlayer grown. With extended growth and longergrowth time, a pyramid shape is eventually formed at the expense of sidefacets, since p-plane exhibits the slowest growth rate. When onlyp-plane terminates the element, the volume growth rate will decrease.This effect is strong when source flows are higher than the p-planegrowth rate allows. This can be used to additional advantage whenfabricating smaller net volume groups at larger spacing, since thenanowire length will determine the pyramid height. The volume ofpyramids can consequently be fully controlled by the nanowire length,when a full pyramid has been formed.

It should be noted that growth rate variations, caused by kineticeffects, such as improved local NH₃ cracking by increased exposure toGaN surface in the group V limited regime, are additional effects to thetheory described above, as the theoretical discussion has been confinedto the contribution surface ratios makes on growth rate when mass flowdominates. However, growth rate response to mass flow and kinetichindrance are not mutually exclusive in any way, except when all kineticbarriers are minimized.

According to another aspect of the present disclosure, a monolithicsubstrate includes an array of emissive subpixels of at least two types,with each type emitting light of a different color. Such a substrate istermed an image chip. With three subpixel types and corresponding colorsthat are approximately red, green, and blue, the subpixel array on theimage chip is capable of displaying full-color images.

The subpixels on the image chip may be electrically-driven two-terminallight-emitters, e.g. semiconductor diodes. The emitters may directlygenerate light of a target color. In this case, the structure andcomposition of the emitter may vary from subpixel to subpixel so thatthe different colors of the image chip can be realized.

Multicolor emission may be achieved using III-Nitride nano-emitters.These are formed by epitaxial growth on a substrate wafer having agrowth mask with vias. During the growth process, deposition is avoidedon the mask, and concentrates in the vias. A variety of structures canbe realized using this approach, including wires, pyramids, as well ascoalesced films.

FIG. 10 illustrates a cross-sectional view of a two-channel cathodeimage chip, which shows the image chip architecture. In this example, a6-channel cathode 120 yields interconnect pitch of about 10 microns. Toform this structure, an isolation etch process, a dielectric fillprocess, a planarization process, and processing steps for formation ofinterconnect structures may be employed.

The substrate 100 may be a sapphire layer, and the channel cathodes 120may include n-type GaN. A multi-channel cathode image chip may befabricated, for example, using the following sequence of steps:

-   1. Deposit subpixel anode material over the structures illustrated    in any of FIGS. 1A-8B, and pattern to keep only where needed to form    the anode contacts (111, 112, 113).-   2. Trench etch the n-type GaN down to the substrate to form isolated    channel cathode lines, which constitute channel cathodes 120.-   3. Fill the trenches with dielectric to form a fill dielectric layer    130.-   4. Planarize the dielectric using e.g. CMP, exposing the tops of the    subpixel anodes.-   5. Deposit and pattern the anode interconnect metal to form anode    interconnect structures 140.-   6. Deposit and pattern a passivation dielectric material (such as    silicon nitride or silicon oxide) to form a passivation layer 150.-   7. Perform a wafer bumping process to form anode bumps 160.

FIG. 11 compares a common-cathode pixel array and a multichannel-cathodepixel array. As far as device fabrication is concerned, the extraprocessing steps for the multichannel cathode may include an isolationetch process, a dielectric fill process, a planarization process, andprocessing steps for formation of interconnect structures. Theprocessing steps for forming the fill dielectric layer 130 and the anodeinterconnect structures 140 are the additional steps employed formanufacture of the multichannel-cathode pixel array compared tomanufacture of the common-cathode pixel array.

The subpixels on the image chip may be electrically-driven two-terminallight-emitters, e.g. semiconductor diodes. The emitters may directlygenerate light of a target color. In this case, the structure andcomposition of the emitter may vary from subpixel to subpixel so thatthe different colors of the image chip can be realized.

Multicolor emission may be achieved using III-Nitride nano-emitters.These are formed by epitaxial growth on a substrate wafer having agrowth mask with vias. During the growth process, deposition is avoidedon the mask, and concentrates in the vias. A variety of structures canbe realized using this approach, including wires, pyramids, as well ascoalesced films.

In one embodiment (FIG. 12), the subpixels include III-Nitridecore-shell nanowires with an InGaN active region, wherein the nanowireis engineered so that a portion of the active region has a bandgapcorresponding to a first, shorter wavelength, and a second portion ofthe wire has a bandgap corresponding to a second, longer wavelength. Insome subpixels, these two portions may be driven in parallel; emissionfrom the small-bandgap long-wavelength active region is energeticallyfavored. Therefore, although both portions of the active region aredriven, light emission from the long-wavelength portion will dominate.In other subpixels, the long-wavelength portion of the wire is removedby a fabrication process, and so these subpixels will generate only theshort-wavelength light. In this way, an array of subpixels emittinglight of different colors can be realized.

In a second embodiment (FIG. 13), the subpixels include III-Nitridenanostructures, wherein the structure and geometry varies from subpixelto subpixel. Such variations may include (1) the height, diameter, orpitch of nanowires, (2) structure variation, e.g. free-standingnanowires in one subpixel type and pyramids in a second subpixel type.In this embodiment, the structural differences are the product of theepitaxial growth process and may be the result of local differences inthe growth mask. Examples of nanostructures that may be formed to emitlight of different wavelengths (e.g., colors) are described above withreference to FIGS. 1A-8B, for example.

Alternately, the subpixel emitters may include a pump that generateslight corresponding to shorter wavelengths and higher energy, and aconversion medium that converts pump light to the final target color.Such a conversion medium may include inorganic phosphor materials,organic luminescent materials, quantum dots, or other semiconductorstructure. Multiple conversion materials may be used in the display toachieve the various-color subpixels in a display. In a display, aconversion material may be used to generate one particular color, whiledirect emission is used to generate additional colors.

On the image chip, each subpixel may be accessed independently using anappropriate combination of contact structures. A circuit diagram of suchan image chip is shown in FIG. 14. In some schemes, each subpixel (101,102, or 103) is provided its own anode contact (111, 112, 113), with thecathode 120 being shared amongst all subpixels in the array. Eachsubpixel (101, 102, or 103) can include a light emitting device, whichmay be a blue light emitting device 101, a green light emitting device102, or a red light emitting device 103.

In other schemes, several subpixels share a single anode contact point,but have independent cathode contacts. An image chip having such adesign is termed a multichannel cathode image chip. A circuit diagram ofa two-channel cathode design is shown in FIG. 15. The multichannelcathodes include at least a first cathode 120A and a second cathode 120Bthat are connected to a same set of anode contacts (111, 112, 113). Inthis multichannel cathode case, the subpixels (101, 102, 103) sharing agiven anode contact (111, 112, 113) are driven in a sequential fashion.The advantage of the multichannel cathode is that the number of contactpoints on the image chip is reduced.

FIG. 16 shows a top-down schematic depiction of several subpixels withinthe array of a two-channel cathode image chip. FIG. 16 illustrates thesubpixel anodes 111, 112, 113, the cathode lines 120 and anodeinterconnects 140, and the metal bumps 160 which serve as the contactpoints for each anode pair.

FIG. 17 shows a cross-section of the two-channel cathode image chip. Thestructure shown in FIG. 17 may be fabricated using the followingsequence of steps:

-   1. Deposit subpixel anode material over the structures illustrated    in any of FIGS. 1A-8B, and pattern to keep only where needed to form    the anode contacts (111, 112, 113).-   2. Trench etch the n-type GaN down to the substrate to form isolated    channel cathode lines, which constitute channel cathodes 120.-   3. Fill the trenches with dielectric to form a fill dielectric layer    130.-   4. Planarize the dielectric using e.g. CMP, exposing the tops of the    subpixel anodes.-   5. Deposit and pattern the anode interconnect metal to form anode    interconnect structures 140.-   6. Deposit and pattern a passivation dielectric material (such as    silicon nitride or silicon oxide) to form a passivation layer 150.-   7. Perform a wafer bumping process to form anode bumps 160.

In one possible modification of this process, the dielectric fill mayconsist of multiple films, including e.g. a dielectric used as ahardmask to facilitate the trench etch of n-type GaN.

Optionally, the original epitaxial substrate may be removed from theimage chip, which may enhance the light-extraction efficiency andminimize waveguiding of light within the image chip. Waveguiding willaffect the local contrast ratio that can be achieved in an image chip,as a portion of the light emitted by a pixel within the array will beextracted at a remote location, giving finite brightness at the remotepixel location even if they are not actively powered. A second variationof the structure uses metal routing that acts as the cathode line, andeliminates the need of the n-GaN semiconductor to carry current. Thiswould allow full singulation of GaN, further reducing waveguiding andimproving image quality.

In an application, the image chip may be mated to a backplane thatdrives the subpixels to display an image. This mating process mayutilize wafer-to-wafer bonding, chip-to-wafer bonding, or chip-to-chipbonding.

All references to top, bottom, base, lateral, etc are introduced for theeasy of understanding only, and should not be considered as limiting tospecific orientation. Furthermore, the dimensions of the structures inthe drawings are not necessarily to scale.

According to various embodiments of the present disclosure, asemiconductor structure including light emitting devices is provided.The light emitting devices include at least a first light emittingdevice (such as nanowire LED 1 in FIG. 7) containing a first nanowire102 b located on a substrate 5. The first light emitting devicecomprises a first active region (such as the combination of a blueemitting region and a green emitting region in FIGS. 12 and 13 orelement 4 in FIG. 7). The first active region includes a first loweractive region (such as a blue emitting region in FIGS. 12 and 13 orelement 4 a in FIGS. 8A and 8B) having a first band gap and the firstupper active region (such as a green emitting region in FIGS. 12 and 13or element 4 c in FIGS. 8A and 8B) having a second band gap. The firstband gap is greater than the second band gap. The light emitting devicesinclude at least a second light emitting device containing a secondnanowire located on the substrate 5. The second light emitting devicecomprises a second active region (such as a blue emitting region inFIGS. 12 and 13) having the first band gap and does not include, nor isin physical contact with, any material having the second band gap.

In one embodiment, the first active region comprises a first III-Vcompound semiconductor material, and the second active region comprisesa second III-V compound semiconductor material. In one embodiment, thefirst III-V compound semiconductor material provides light emission at afirst peak emission wavelength, and the second III-V compoundsemiconductor material provides light emission at a second peak emissionwavelength that is longer than the first peak emission wavelength.

In one embodiment, the first nanowire comprises a first growth template102 b including a first nanowire core, the second nanowire comprises asecond growth template 102 a including a second nanowire core, and thefirst nanowire core and the second nanowire core comprise asemiconductor material having a first conductivity type. The firstconductivity type can be p-type or n-type. The first lower active region(such as 4 b) can be located around a lower portion of the firstnanowire core and comprises at least one first quantum well that has thefirst band gap. The first upper active region (such as 4 c) can belocated around an upper portion of the first nanowire core and comprisesat least one second quantum well that has the second band gap. Thesecond active region can be located around the second nanowire core andcomprises at least another first quantum well that has the first bandgap. In one embodiment, the first nanowire core and the second nanowirecore can comprise gallium nitride, the at least one first quantum welland the at least another first quantum well can comprise indium galliumnitride having the first band gap, and the at least one second quantumwell can comprise indium gallium nitride having the second band gap.

In one embodiment, a growth mask 6 can be located on the substrate 5.The growth mask 6 can include a first aperture and a second aperturetherein. The first nanowire core can be epitaxially aligned to a singlecrystalline semiconductor material of the substrate. The second nanowirecore can be epitaxially aligned to the single crystalline semiconductormaterial of the substrate. In one embodiment, the light emitting devicescan comprise at least one first growth template located around the firstnanowire core and extending laterally beyond the first aperture over thegrowth mask, and at least one second growth template located around thesecond nanowire core and extending laterally beyond the second apertureover the growth mask.

In one embodiment, the first growth template and the second growthtemplate differ from each other by at least one of: a respective growtharea defined by an area of the first aperture and an area of the secondaperture, respectively; a respective facet area for a same type ofcrystallographic plane; and a respective nearest-neighbor spacing fromadjacent growth templates that are present on the substrate. In oneembodiment, the first aperture has an area that is at least twice thearea of the second aperture.

In one embodiment, the light emitting devices can further comprise afirst junction forming element 3 having a doping of a secondconductivity type that is the opposite of the first conductivity typeand located around the first active region to provide a first junctionselected from a p-n junction and a p-i-n junction, and a second junctionforming element having a doping of the second conductivity type andlocated around the second active region to provide a second junctionselected from a p-n junction and a p-i-n junction.

In one embodiment, the first growth template can comprise a III-Vcompound semiconductor material and can have m-planes as sidewalls andp-planes as faceted top surfaces, and the second growth template cancomprise the III-V compound semiconductor material and has m-planes assidewalls and a generally horizontal top surface.

In one embodiment, the first growth template comprises a III-V compoundsemiconductor material and has m-planes as sidewalls and p-planes asfaceted top surfaces, and the second growth template comprises the III-Vcompound semiconductor material and has p-planes as faceted surfacesthat are adjoined at an apex. In one embodiment, the first nanowire coreis epitaxially aligned to a single crystalline semiconductor material ofthe substrate, the second nanowire core is epitaxially aligned to thesingle crystalline semiconductor material of the substrate, and a bottomperiphery of the p-planes of the second growth template contacts a topsurface of the growth mask.

In one embodiment, the light emitting devices can include a third lightemitting device comprising a nanopyramid growth template 2 c located onthe substrate (as illustrated in FIGS. 3A and 3B), and a third activeregion (e.g., a red emitting region) having a third band gap that isless than the second band gap. The growth mask 6 can include a thirdaperture therein. The first nanowire core can be epitaxially aligned toa single crystalline semiconductor material of the substrate, the secondnanowire core can be epitaxially aligned to the single crystallinesemiconductor material of the substrate, and the nanopyramid growthtemplate can be epitaxially aligned to the single crystallinesemiconductor material of the substrate.

In one embodiment, additional light emitting devices can be present onthe substrate around each of the first, second, and third light emittingdevices. The third aperture can have an area that is substantially thesame as second aperture. As used herein, two areas are substantially thesame if each area is with 20% of the average of the two areas. Thenearest-neighbor spacing of the third light emitting device from otherlight emitting devices of the semiconductor device is greater thanrespective nearest-neighbor spacings of the first and second lightemitting devices.

In one embodiment, the first growth template comprises a III-V compoundsemiconductor material and has m-planes as sidewalls and p-planes asfaceted top surfaces, the second growth template comprises the III-Vcompound semiconductor material and has p-planes as faceted surfacesthat are adjoined at an apex, the nanopyramid growth template comprisesthe III-V compound semiconductor material and has p-planes as facetedsurfaces that are adjoined at an apex; the semiconductor structurefurther comprises a growth mask located on the substrate, and a bottomperiphery of the p-planes of the nanopyramid growth template contacts atop surface of the growth mask as illustrated in FIGS. 3A and 3B.

In one embodiment, the light emitting devices constitute atwo-dimensional pixel array as illustrated in FIGS. 14-16. Each pixel inthe two-dimensional pixel array can include one instance of the firstlight emitting device, one instance of the second light emitting device,and one instance of the third light emitting device.

According to various embodiments of the present disclosure, a method ofmaking a semiconductor device comprising light emitting devices isprovided. A growth mask 6 having a first aperture in a first area and asecond aperture in a second area is formed on a substrate 5. A firstnanowire is formed through the first aperture. The first nanowirecomprises a first active region including a first lower active regionand a first upper active region. The first nanowire 8 b comprises afirst active region (such as the combination of a blue emitting regionand a green emitting region in FIGS. 12 and 13 or element 4 in FIG. 7).The first active region includes a first lower active region (such as ablue emitting region in FIGS. 12 and 13 or element 4 a in FIGS. 8A and8B) having a first band gap and the first upper active region (such as agreen emitting region in FIGS. 12 and 13 or element 4 c in FIGS. 8A and8B) having a second band gap. The first band gap is greater than thesecond band gap. A second nanowire is formed through the secondaperture. The second nanowire comprises a second active region (such asa blue emitting region in FIGS. 12 and 13) having the first band gap anddoes not include, nor is in physical contact with, any material havingthe second band gap.

The first nanowire and the second nanowire may be provided by forming afirst growth template comprising a III-V compound semiconductor materialthrough the first aperture, and forming a second growth templatecomprising the III-V compound semiconductor material through the secondaperture. The first active region and the second active region can beformed by simultaneous deposition of III-V semiconductor materialshaving different compositions. The first active region can comprise afirst III-V compound semiconductor material that provides light emissionat a first peak emission wavelength, and the second active region cancomprise a second III-V compound semiconductor material that provideslight emission at a second peak emission wavelength that is longer thanthe first peak emission wavelength.

A first nanowire core and a second nanowire core may be formed byepitaxially growing a semiconductor material having a first conductivitytype on the substrate. The first lower active region, the first upperactive region, and the second active region may be formed simultaneouslyby growing another semiconductor material. The first lower active regionmay be formed on, and around, a lower portion of the first nanowire coreand comprises at least one first quantum well that has the first bandgap. The first upper active region may be formed on, and around, anupper portion of the first nanowire core and comprises at least onesecond quantum well that has the second band gap. The second activeregion may be formed on, and around, the second nanowire core andcomprises at least another first quantum well that has the first bandgap.

In one embodiment, the first nanowire core and the second nanowire coreare formed by growing gallium nitride on the semiconductor substrate.The first quantum wells of the first and second light emitting devicesmay be formed by depositing indium gallium nitride having the first bandgap. The second quantum well may be formed by depositing indium galliumnitride having the second band gap. The first quantum wells and thesecond quantum wells may be simultaneously deposited with differentcomposition due to differences in the deposited material while employinga same set of deposition precursors, where the differences in the bandgaps between the first quantum well and the second quantum well may beproduced by the differences in at least one of a respective growth areadefined by an area of the first aperture and an area of the secondaperture, respectively; a respective facet area for a same type ofcrystallographic plane; and a respective nearest-neighbor spacing fromadjacent growth templates that are present on the substrate.

An image chip comprising a semiconductor structure of the presentdisclosure is also provided. In one embodiment, the semiconductorstructure may include a substrate, such as a monolithic substrate, andthe light emitting devices may constitute an array of emissive subpixelsof at least two types located upon the monolithic substrate. Inaddition, the methods of the present disclosure may be employed tomanufacture an image chip that includes the semiconductor structure.

While the disclosure has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the disclosure is not to be limited to thedisclosed embodiments, on the contrary, it is intended to cover variousmodifications and equivalent arrangements within the scope of theappended claims.

The invention claimed is:
 1. A display device including light emittingdevices, comprising: a first light emitting device containing a firstsemiconductor active region located over a substrate, wherein the firstsemiconductor active region has a first band gap that emits light at afirst wavelength; and a second light emitting device containing a secondsemiconductor active region located over the substrate and a conversionmedium overlying the second semiconductor active region, wherein theconversion medium converts shorter wavelength light emitted from thesecond semiconductor active region to light having a target color;wherein: the first semiconductor active region is located in a firstnanowire; the second semiconductor active region is located in a secondnanowire; the second semiconductor active region has the first band gap;the conversion medium comprises an inorganic phosphor, organicluminescent material or quantum dots; and the conversion mediumgenerates the target color having a longer wavelength than the shorterwavelength light emitted from the second semiconductor active region,and direct emission from the first light emitting device generates anadditional color to achieve different color subpixels in a display.